Evidence if mounting that the reported April 4 chemical weapons attack in the northwestern Syrian province of Idlib may have been staged, Lavrov said.

"There is growing evidence that this was staged, meaning the incident with the use of chemical weapons in Idlib province."

He cited expert opinion publicized in the West stating that "there are too many inconsistencies, discrepancies in the version used to justify the April 7 aggressive action," referring to the US Tomahawk missile attack on the Sha'irat airfield.

The three countries insist that the US and its allies must respect Syria's sovereignty, Lavrov said, stressing that there must be a careful investigation of the chemical weapons incident in Idlib under OPCW auspices.

Russia's proposal on the investigation of Idlib chemical attack is being met with resistance, Lavrov said.

Lavrov added that any reaction from the OPCW on the data provided by Moscow and Damascus concerning militants' use of chemical weapons in Aleppo was absent.

Both Russia and Iran favor an OPCW investigation into the reported chemical attack in Idlib with the participation of specialists from different countries, Lavrov said, adding that claims of Moscow not trusting the organization are false as Russia just wants the probe to be transparent.

Lavrov said that the US missile attack on a Syrian airfield was aimed at the regime change.

Syrian Foreign Minister then called the trilateral meeting with Lavrov and Zarif a 'strong message' after the US strike in Syria.

"We shall facilitate everything which will lead to the success of the commission’s work on investigation. Our question is as follows, why are the United States and its allies afraid of such commission being established? If the US strike has no justification, it is blatant aggression against Syria’s sovereignty," Muallem told reporters in Moscow.

The Foreign Minister of Iran agreed with his Russian and Syrian counterparts on the need to boost cooperation in fighting terror, as well as carrying out the chemical attack probe.

Zarif said that chemical weapons cannot be used as a pretext for any actions against sovereignty and territorial integrity of an independent state.

"The use of chemical weapons as a pretext for taking actions against the sovereignty and territorial integrity of an independent state and a UN member is very dangerous. The international investigation is required in this case… It is necessary to prevent such actions in the future."